A magnetic resonance imaging apparatus is an apparatus configured to excite a nuclear spin of a test object put in a static magnetic field by an RF (radio frequency) signal of a Larmor frequency, and to reconstruct a magnetic resonance signal generated by the test object as the test object is excited so as to generate an image.
A magnetic resonance imaging apparatus of a high static magnetic field strength, e.g., 3 T (Tesla) recently comes into wide use. A raised static magnetic field strength increases an SNR. Then, space or time resolution enhanced by an advantage of a high SNR is expected.
In the meantime, the raised static magnetic field strength is accompanied by growth in unevenness of the static magnetic field. A method called SSFP (Steady State Free Precession) is particularly sensitive to the unevenness of the static magnetic field. An imaging operation using an SSFP method has a problem of a band-shaped artifact called a banding artifact, e.g., as disclosed in Japanese Unexamined Patent Publication No. 2011-167559.
The SSFP method is doomed to suffer a phase mismatch in a pixel in the presence of unevenness of the magnetic field. Although a slight phase mismatch causes no significant problem, the phase mismatch enlarges if unevenness of the magnetic field enlarges as the magnetic field strength increases. If the phase mismatch reaches 180 degrees, positive and negative signals cancel each other resulting in a relevant displayed pixel looking black. This phenomenon is typically called a banding artifact which undesirably appears on an image. The banding artifact takes a form of band-shaped artifacts appearing periodically in space, where an appearance interval broadens as a repetition period of time (TR interval) of an excitation RF pulse shortens, and conversely narrows as the repetition period of time TR lengthens.
One method for reducing the banding artifact is to shift a center frequency F0. The banding artifact appears where the phase mismatch is 180 degrees. Thus, conditions of the phase mismatch can be changed by means of a shift in the center frequency F0. If the conditions of the phase mismatch change, a position at which a banding artifact appears on the image shifts correspondingly to that change.
For capturing an image of the heart, a banding artifact appearing at a position excepting the heart is not a significant problem. Meanwhile, a targeted region of interest (ROI) such as the heart is not very large. Thus, a shift in the center frequency FO for changing the position of appearance of the banding artifact can make it possible to avoid a bad influence on the image of the heart. A center frequency F0 at which an influence of the banding artifact can be avoided is called an “optimum F0”, hereafter.
The influence of the unevenness of the static magnetic field differs on a patient-by-patient basis, and differs depending upon a position or orientation of an imaging cross section even of one and the same patient. Thus, a usual practice is to have a preparatory scan to search for an optimum F0 before and independently of a main scan (a scan for capturing an originally intended image) in order to search for an optimum F0 for every patient.
Imaging conditions for a preparatory scan are ordinarily set for the preparatory scan to search for an optimum F0 separately from imaging conditions for a main scan. An operation load for the preparatory scan is so large to take time. Further, an ordinary practice of the preparatory scan is such that a user determines an optimum F0 from images less affected by the banding artifact while manually changing and displaying plural images obtained in the preparatory scan. Then, the user sets the optimum F0 as one of imaging conditions for a main scan on another occasion of starting the main scan. Thus, it takes time to determine the optimum F0, and it takes time for an operation to reflect the optimum F0 on the main scan as well.
In order to solve the above problems, a magnetic resonance imaging apparatus by which an optimum F0 for avoiding a banding artifact can be efficiently determined with a small operation load is demanded.